1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type or electrostatic recording type embodied as a copying machine, a printer and the like, and a developer density controlling apparatus used with such an image forming apparatus, and more particularly it relates to an image forming apparatus having a density controlling apparatus for controlling toner density or image density of the developer developing agent through toner replenishing control of two-component developer.
2. Related Background Art
In general, in developing devices of an image forming apparatuses of electrophotographic type or electrostatic recording type, one-component developer mainly including magnetic or non-magnetic toner, or two-component developer mainly including non-magnetic toner and magnetic carrier is used. Particularly, in color image forming apparatuses for forming a full-color or multi-color image by an electrophotographic system, many developing devices utilize two-component developer in consideration of color tone and image quality of an image.
As is well-known, toner density (ratio of a weight of toner with respect to a total weight of carrier and toner) of the two-component developer is a very important factor for stabilizing image quality. During development, the toner in the developer is gradually consumed to reduce the toner density. Thus, it is required that the toner density or image density is always detected by using a toner density controlling apparatus (toner density detecting means) or image density detecting means and toner is replenished in accordance with the change in density to keep the toner density or image density constant, thereby maintaining the image quality.
FIG. 7 shows an example of an image forming apparatus (digital copying machine of electrophotographic type in this example) having a conventional toner density controlling apparatus.
First of all, an image on an original G is read by a CCD 1, and a read analogue image signal is amplified to a predetermined level by an amplifier 2 and then is converted into an 8-bit (0 to 255 gradations) digital image signal by an analogue/digital converter (AID converter) 3, for example.
Then, the digital image signal is sent to a xcex3-converter 5 (converter for effecting density conversion by a look-up table system constituted by a 256-byte RAM, in this example), where the signal is subjected to xcex3-correction. Thereafter, the signal is inputted to a digital/analogue converter (D/A converter) 9.
The digital image signal is converted into an analogue image signal again by the converter 9 and then is inputted to one of inputs of a comparator 11. A triangular wave signal having predetermined period generated from a triangular wave generating circuit 10 is inputted to the other input of the comparator 11, so that the analogue image signal supplied to the one input of the comparator 11 is compared with the triangular wave signal and is subjected to pulse width modulation. The pulse width modulated binary image signal is inputted to a laser driving circuit as it is, and is used as an ON/OFF control signal for light emission of a laser diode 13. A laser beam emitted from the laser diode 13 is scanned by a known polygon mirror 14 in a main scanning direction and is illuminated onto a photosensitive drum 40 as an image bearing member (rotated in a direction shown by the arrow) through an fxcex8 lens 15 and a reflection mirror 16, thereby forming an electrostatic latent image.
On the other hand, the photosensitive drum 40 is subjected to uniform electricity removal by an exposure device 18 and then is uniformly charged, for example negatively, by a primary charger 19. Thereafter, the laser beam is illuminated onto the photosensitive drum, thereby forming the electrostatic latent image corresponding to an image signal. The electrostatic latent image is developed by a developing device 20 as a visualized image (toner image). A toner replenishing hopper (tank) 8 containing replenishing toner 29 is attached to an upper part of the developing device 20, and a toner carrying screw (toner replenishing means) 30 rotated by a motor to carry the toner 29 and supply it into the developing device 20 is disposed at a lower part of the hopper 8.
The toner image formed on the photosensitive drum 40 is transferred, by a transfer charger 22, onto a transfer material P conveyed to the photosensitive drum 40 by a transfer material bearing belt 17. The transfer material bearing belt 17 is mounted and extending between two rollers 25a and 25b and is driven in a direction shown by the arrow in an endless fashion; meanwhile, the transfer material P borne on the belt is conveyed to the photosensitive drum 40. Residual toner remaining on the photosensitive drum 40 is scraped by a cleaner 24.
Incidentally, for simplifying the explanation, although only a single image forming station (comprised of latent image forming means including the exposure device 18 and the primary charger 19, the photosensitive drum 40, the developing device 20 and the like) is shown, in case of a color image forming apparatus, image forming stations corresponding to various colors (for example, cyan, magenta, yellow and black) are arranged in series above the transfer material bearing belt 17 along the shifting direction thereof.
The image forming apparatus is designed so that, in order to keep the toner density of developer or image density constant by effecting control for replenishing the toner to the developer 21 (toner density thereof is decreased) within the developing device 20, by controlling rotation of a motor 28 by a CPU 6 through a motor driving circuit 7 on the basis of an output value a toner density sensor 23 provided within the developing device 20, the control for replenishing the toner to the developer 21 within the developing device 20, thereby keeping the toner density of developer or image density constant. Control data supplied to the motor driving circuit 7 is stored in a RAM 90 connected to the CPU 6.
In order to control the toner density of developer or image density to be kept constant by effecting control for replenishing the toner to the developer 21 (toner density thereof is decreased) within the developing device 20, one of density controlling apparatuses (ATR) of various types is provided within the image forming apparatus.
More specifically, regarding the toner density sensor 23 provided within the developing device 20, an auto-toner replacement (ATR) control system (developer reflection ATR) in which the toner density of the developer 21 within the developing device 20 is detected as a reflected light amount, a control system (inductance ATR) in which permeability of magnetic carrier of the developer 21 within the developing device 20 is detected, or a control system (patch detection ATR) in which a reference patch image 26 is formed on the photosensitive drum 40 and image density of the patch image is detected by a sensor 27 such as a potential sensor opposed to the photosensitive drum 40 can be used.
However, since each of the above-mentioned ATRs by itself detects only the image density or toner density, countermeasure to change in environment or change in developing ability due to degradation of developer cannot be effected, so that the image may be deteriorated. Thus, a technique in which two or more ATRs are combined to compensate for the respective defects thereby to permit toner replenishing control has been proposed, as disclosed in Japanese Patent Application Laid-Open No. 9-127757.
According to this patent document, the toner is replenished by the developer reflection ATR, and, in this case, a toner excess/deficiency amount required for returning patch image density to initial density is calculated from output signals representative of difference in density of the patch image, and the toner replenishing amount is corrected by adding or subtracting the calculated result with respect to a target value set in the developer reflection ATR so that the toner replenishing control of the developer reflection ATR is effected by using the corrected toner replenishing amount, thereby preventing overflow and/or fog of the developer, while stabilizing the image density.
However, the above-mentioned combined density controlling apparatus has the following disadvantages.
For example, if the toner density in developer becomes greater than the target value of the developer reflection ATR due to a reading error of the developer reflection ATR and/or dispersion in toner replenishing amount of the toner replenishing hopper, the patch detection ATR will judge that the image density is high and decrease the target value of the developer reflection ATR excessively. Consequently, even when the image density becomes proper by the toner consumption during the copying operation, since the target value of the developer reflection ATR is decreased, the toner is not replenished until the toner is further consumed to decrease the image density. That is to say, if the image density is once deviated greatly for any reason, the change in image density will continue for a long term.
Further, in such a condition, if images which consume less toner continue to be copied, since the excessive toner cannot be consumed, the target value of the developer reflection ATR is abruptly decreased so that the difference between the toner density during the development and the target value becomes great, with the result that the control becomes impossible, or, in a system having control for detecting malfunction and/or erroneous detection of the respective ATR sensors, abnormality of the ATR sensor may be detected notwithstanding the ATR sensors are operated correctly.
An object of the present invention is to provide a developer density controlling apparatus and an image forming apparatus, in which image density can be controlled stably.
Another object of the present invention is to provide a developer density controlling apparatus and an image forming apparatus, in which, even if toner density in developing means is greatly deviated from a target value, image density can be controlled stably to obtain high quality images from initiation of image formation.
A further object of the present invention is to provide a developer density controlling apparatus comprising developer density detecting means for detecting toner density in developer, toner replenishing means for replenishing toner on the basis of difference between the toner density detected by the developer density detecting means and target density, and image density detecting means for detecting image density of a reference image formed and for correcting the target density on the basis of the detected image density, wherein, if the difference between the toner density detected by the developer density detecting means and the target density is equal to or greater than a predetermined value, the image density detecting means do not correct the target density.
A still further object of the present invention is to provide an image forming apparatus comprising an image bearing member for bearing a latent image; and a developer density controlling apparatus including developer density detecting means for detecting toner density in developer, toner replenishing means for replenishing toner on the basis of difference between the toner density detected by the developer density detecting means and target density, and image density detecting means for detecting image density of a reference image formed and for correcting the target density on the basis of the detected image density, wherein, if the difference between the toner density detected by the developer density detecting means and the target density is equal to or greater than a predetermined value, the image density detecting means do not correct the target density.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.